Project Summary Title: Exploiting mitochondrial heteroplasmy for cancer chemotherapy Roughly a third of patient cancers are heteroplasmic -- that is, individual cells harbor a mixture of genetically distinct mitochondrial genomes -- and a substantial fraction of these bear severe loss-of-function mutations affecting genes necessary for respiration. These mutations appear to be passengers rather than drivers of tumorgenesis, but our lab recently discovered that they can render cancer cell lines and xenografts more vulnerable to biguanides, mitochondrial inhibitors used to treat type 2 diabetes. Since heteroplasmy is relatively rare in normal tissues, these findings suggest that mitochondrial inhibitors may have a therapeutic window for treating heteroplasmic cancers, but specifically when and how this heteroplasmy may be exploited for treatment remains poorly understood. Additionally, our work showed that heteroplasmy is a reversible genetic defect, since heteroplasmic cells generally still contain wild-type copies of the mitochondrial genome, and that partial reversion is a route to drug resistance. This reduction of heteroplasmy is not a simple mutational processes, and its mechanisms are unknown. The proposed work aims to fill both of these gaps in current knowledge with a systematic study of how heteroplasmy affects susceptibility to a variety of relevant inhibitors and how these inhibitors may drive changes in heteroplasmy leading to drug resistance.